[oh-my-zsh]$ZSH should be used to compose its install directory

[SteveLauC]

When determining if oh-my-zsh is installed, we check the existence of its installation directory, and this directory can be customized through the $ZSH environment variable when installing, which means the directory is possibly not ~/.oh-my-zsh. However, in the current impl of topgrade, it is hardcoded to ~/.oh-my-zsh:

topgrade/src/steps/zsh.rs

Lines 156 to 157 in 14753a1

	require("zsh")?;
	let oh_my_zsh = HOME_DIR.join(".oh-my-zsh").require()?;

We should do something like this:

    let oh_my_bash = var("ZSH")
        // default to `~/.oh-my-zsh`
        .unwrap_or(
            HOME_DIR
                .join(".oh-my-zsh")
                .to_str()
                .expect("should be UTF-8 encoded")
                .to_string(),
        )
        .require()?;